The present invention relates to a compensating circuit and an equalizer for correcting or compensating a positive-negative asymmetry of a reproduced signal outputted from a magnetoresistive head or the like. Further, the invention is concerned with a magnetic recording/reproducing apparatus using the compensating circuit and the equalizer.
In the field of the magnetic disk apparatus, there is an increasing trend of implementing a reproducing head with high sensitivity in an effort to realize a high-density package, which has led to a development of a reproducing head based on magnetoresistive effect or giant magnetoresistive effect. Hereinafter, these head will be referred to also as the MR head. The MR head is now replacing the conventional inductive head.
For having better understanding of the invention, description will be made in some detail of the technical background thereof. The non-linearity of the MR head will be briefly reviewed by reference to FIG. 8. The MR head is so designed as to perform a signal reproduction by making use of a magnetic field-versus output conversion characteristic curve 76 of a magnetoresistive element such as illustrated in FIG. 8. The MR head is implemented such that a bias field is applied to the magnetoresistive element for determining which portion of the magnetic field-versus-output conversion characteristic curve 76 is to be used. More specifically, the bias field 73 is superposed on the input field 71 applied from a magnetized recording medium so that an active region 74 is established on a linear portion of the magnetic field-versus-output conversion characteristic curve 76. To this end, there have been proposed a head structure in which the active region is controlled by a magnetic field generated by the bias current or by making use of the interlayer coupling (or magnetic interaction) among a sense current applied to the magnetoresistive element, magnetization of the magnetoresistive element and the bias field generated by a biasing magnetic layer. For this reason, non-linearity represented typically by the positive-negative asymmetry (i.e., asymmetry of output waveform with reference to a given level) may make appearance in the reproduced waveform 72 due to deviation of the bias current, dispersion of the magnetic characteristic of the magnetoresistive elements and other factors. In general, the signal processing system is designated on the presumption that the magnetic recording/reproducing system is of linear property. Thus, the asymmetry presents a very influential factor for degradation of the system performance. Under being the circumstances, there has been proposed a method of protecting the system against performance degradation mentioned above by compensating (or correcting) the non-linearity before the signal outputted from the MR head is inputted to an equalizer. According to the compensating method in which non-linearity compensating analogue-to-digital conversion (hereinafter referred to as the A/D conversion) is adopted, the scale of an A/D converter is set to one of different values in dependence on whether the input signal to the A/D converter assumes positive or negative polarity, for thereby compensating the positive-negative asymmetry of the reproduced waveform. For more particular, refer to Japanese Unexamined Patent Application Publication No. 205205/1993 (JP-A-5-205205). Further, according to an adaptive digital linearization method and apparatus for compensating the asymmetry of the output signal of the MR head (refer to Japanese Unexamined Patent Application Publication No. 44510/1994 (JP-A-6-44510)), amplitude values of the reproduced signal are compensated or collected by reference to a look-up table after the A/D conversion of the reproduced signal. To this end, the characteristics of the individual MR heads are previously measured by changing the bias field applied thereto, and when the characteristic of the MR head changes, compensation therefor is effectuated by correcting the amplitude value by referencing the look-up table as stored. Additionally, as another method of correcting or compensating the non-linearity of the reproduced signal before being inputted to the equalizer, there may be mentioned a method of canceling out the non-linearity by approximating the characteristic reverse to the magnetic field-versus-output voltage conversion property with a degree-N function (where N=3 or so). Besides, there has also been proposed a method of suppressing degradation due to the non-linearity by optimizing the equalization target of a FIR filter (finite impulse response filter. Reference may be made to Japanese Unexamined Patent Application Publication No. 266403/1993 (JP-A-5-266403)). Moreover, such a method is also known according to which non-linearities inclusive of non-linear transition shift (bit shift) by resorting to the use of an equalizer constituted by a combination of a decision feedback equalizer and a RAM (random access memory).
The scheme of canceling out or compensating the non-linearity of the output signal of the MR head before being inputted to the equalizer is certainly effective. However, in order to cancel out the non-linearity satisfactorily or completely, there will be demanded a canceler of complicated characteristics, realization of which needs arithmetic circuits of complicate structure and/or a memory of large capacity, leading to increasing in the system scale and the manufacturing cost. Further, the method of compensating the non-linearity of the output of the MR head by using the A/D converter itself is difficult to realize in an LSI (Large Scale Integrated circuit) with high accuracy because the voltages applied to the components of the A/D converter circuit and the load will change upon regulation of the reference voltage for changing the scaling. Besides, in the method mentioned just above, the asymmetry compensation coefficient (or correcting coefficient) is set in terms of an analogue value of the reference voltage for the A/D converter. Consequently, due to dispersions of the circuit elements and other factors, difficulty will be encountered in realizing the compensation of the non-linearity of the output signal of the MR head.
At this juncture, it should however be mentioned that the compensation of the non-linearity of the output signal of the MR head is basically to improve the so-called error rate, for which perfect or complete cancellation of the non-linearity is not necessarily demanded. The portion of the characteristic curve of the MR head which exerts most remarkable influence to the error rate is the portion where distortion is extremely large. In this conjunction, it is noted that when the positive and negative amplitudes of the waveform of the nonlinear output of the MR head are asymmetrical, the distortion mentioned above makes appearance most remarkably in the signal portion where the amplitude thereof is large. Accordingly, by performing the amplitude compensation or correction for such signal portion, it is possible to improve the error rate with enhanced efficiency.